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Accomplishments of Astronomy and Astrophysics Explorers 16 TABLE 1 ASTROPHYSICS EXPLORERS Sequence PAST MISSIONS Date of Launch* Dedicated Astrophysics Missio118 1. Explorer 11 ')'-Ray Astronomy 1961 SAS-1: X-Ray ( Uhuru) 2. Explorer 38 RAE-1: Radio Astronomy 1968 3. Explorer 42 1970 4. Explorer 48 SAS-2: -y-Ray 1972 5. Explorer 49 RAE-2: Radio Astronomy 1973 6. Explorer 53 SAS-3: X-Ray 1975 7. IUE International Ultraviolet Explorer 1978 8. IRAS Infrared Astronomy Satellite 1983 Participation in Other Missions 1. IMP Series Cosmic-Ray Experiments 1963-1973 2. Ariel 5 X-Ray Monitor 1974 3.ANS X-Ray Proportional Counters 1974 4. ISEE-3 Cosmic-Ray and Gamma-Ray Experiments 1978 FUTURE MISSIONS Dedicated Astrophysics Missio118 1. COBE Cosmic Background Explorer 1989 2. ElNE Extreme Ultraviolet Explorer 1990 3.XTE X-Ray Timing Explorer 1991 Participation in Other Missions 1. ROSAT High Resolution Imager: Roentgen Satellite 1987 2.HNC Heavy Nuclei Collector on Long Duration 1987 Exposure Facility 3. CRRES Cosmic-Ray Isotope Studies: 1987 Combined Release/R adiation Effects Satellite ⢠Launch dates ror ruture miaaions a.re listed a.a they were planned prior to the inter ruption or Shuttle launches. Delays or at least two years, and in many cues signifi cantly longer, a.re likely.
Accomplishments of Astronomy and Astrophysics Explorers 17 4U1538-52 SMCX-1 Cen X- 3 Her X- I PSR 1913+16 0 Neutron- Star Moss !M0) SAS-9: Masses or neutron stars observed via their x-ray emission by SAS-3 (upper six ) and via radio emission (the binary pulsar PSR 19 13+ 16). T he hatched band shows the range or masses eljP ected rrom theoretical considerations or neutron-star rormation, namely, 1.2 - 1.6 l. 1Y.I() events with possible gamma-ray signatures were recorded from the sky, and their rate of occurrence implied a firm upper limit on the intensity of cosmic gamma rays. This measurement disproved one version of the steady state theory of cosmology whereby nucleon-antinucleon pairs were assumed to be continu ously created everywhere so as to maintain a constant density in the expanding universe.
Accomplishments of Astronomy and Astrophysics Explorers 18 EXPLORER 38: RAE-1 Radio Astronomy Explorer 1 ( RAE-1 ), the first spacecraft designed specifically for radio astronomical studies, provided measurements over the frequency range 200 kHz to 9.2 MHz. Although the most extensive results from this mission were directed towards solar system astronomy and space plasma physics of the terrestrial magnetosphere, observations were made of galactic sources. Continuum background maps at fre quencies of 4 and 6 MHz defined the distribution of the ionized component of the interstellar medium. Cosmic noise back ground spectra measured down to 0.5 MHz provided new esti mates of the interstellar flux of cosmic rays, of magnetic fields in the galactic halo, and of the nature of radiation from distant extragalactic radio sources. EXPLORER 42: SAS- 1 (UHURU) The launch of Uhuru ( Small Astronomy Satellite: SAS) , marked the first of the small satellites devoted to x-ray astron omy. It provided the first comprehensive sky survey in x rays and discovered several hundred galactic and extragalactic sources. Positions of these sources were measured with suffi cient accuracy to enable identifications of optical counterparts. Uhuru also discovered pulsating x-ray stars in binary systems that are rotating, accreting, magnetized neutron stars, and unique temporal variations in Cygnus X-1 supporting the idea that Cygnus X-1 is a black hole in a close binary system. Extragalactic sources revealed variable x-ray emission from active galactic nuclei. Hot intergalactic plasma was found in certain clusters of galaxies with a total mass comparable to the total visible mass of the galaxies in the cluster. EXPLORER 48: SAS-2 The second Small Astronomical Satellite ( SAS-2 ) , carrying a '"'{-ray telescope, confirmed the discovery of galactic and
Accomplishments of Astronomy and Astrophysics Explorers 19 extragalactic gamma rays by the Orbiting Solar Observatory 3 (OS0-3), and mapped the intensity of high-energy gamma rays over 60 percent of the sky including most of the galactic ........- plane. The distribution of ')-ray emission appeared generally correlated with galactic structural features, including spiral arm segments, as expected for cosmic-ray interactions with the interstellar gas. The uniformity measurements of diffuse ') radiation clearly eliminated the galactic halo as the primary source. The intensity, uniformity, and energy spectra when combined seem to limit the possible origins to two: namely, active galaxies or matter-antimatter interactions at the boun daries of superclusters of galaxies in a baryon symmetric universe. Observations of some identified point sources showed pulsations or variability; in addition optically unidentified strong "f-ray sources were discovered. EXPLORER 49 : RAE-2 The second spacecraft dedicated to observations at radio frequencies 25 kHz to 13 MHz, the RAE-2 was launched into lunar orbit to reduce contamination from terrestrial radio emis sions. Its orbit resulted in repeated lunar occultations of the Earth providing the welcome shielding of Earth's emissions and enabling galactic measurements to be made without interfer ence. New measurements of the nonthermal galactic radio spectrum at frequencies below 10 MHz and survey maps of the spatial distribution of the low frequency galactic emission were obtained. The lowest frequencies allowed construction of a coarse picture of the local interstellar medium and its magnetic field in the immediate solar neighborhood. EXPLORER 53: SAS-3 SAS-3 was an x-ray observatory spanning the energy range from 0.1 to 50 keV. Among its results were the discovery and analysis of the Rapid Burster and many other new x-ray burst sources. The observations led to the conclusion that a
Accomplishments of Astronomy and Astrophysics Explorers 20 burster is a weakly magnetized neutron star accreting matter from a low mass companion, and to the identification of two >,ypes of x-ray bursts of which one is caused by fluctuations in the accretion flow, and the other by thermonuclear flashes in material accumulated on the neutron star surface. Timing stu dies of binary x-ray pulsars, including several discovered with SAS-3, yielded numerous precise orbit determinations and ::trong evidence that the masses of the neutron stars in such systems all lie within a narrow range around the Chan drasekhar mass limit for white dwarfs. Position determinations of over 60 galactic and extragalactic x-ray sources led to more than 40 new optical identifications including a nearby quasar, the hot white d warf HZ43, and the first counterpart of an x-ray burster. An all-sky survey of the soft (0.1-0.3 keV) x-ray back ground provided evidence that a substantial portion originates in sources beyond the neutral hydrogen of the galactic disk and may be thermal emission from the galactic halo. With SAS-3 a Guest Observer program was initiated for Explorers. This program allowed scientists, other than the Principal Investigator's team, to perform observations with the satellite. The Guest Observer program signaled the beginning of a rewarding new phase in space research with Explorers. INTERNATIONAL ULTRA VIOLET EXPLORER The International Ultraviolet Explorer (IUE) containing a small (0 45m diameter) ultraviolet telescope and spectrograph . in geosynchronous orbit was built by NASA, the European Space Agency (ESA), and the Science and Engineering Research Council (SERC) of the United Kingdom. The observing time and spacecraft control are shared by the sponsoring agencies. NASA assigns two-thirds of the observing time; ESA/SERC scientists are allocated one-third. All of the observing time is allocated by peer review to Guest Observers forming a large international scientific community. About 21 0 programs are accepted by NASA alone each year; most of these programs involve several researchers. ESA/SERC additionally accepts many proposals for their observing time.
Accomplishments of Astronomy and Astrophysics Explorers 21 Since launch in 1978, the IUE continues to produce ultra violet spectra of solar system, galactic, and extragalactic objects. Scientific programs are varied. Spectra from IUE have revealed a hot halo surrounding our galaxy and variability ascribed to different regions within active galactic nuclei. Observations of stars have demonstrated that winds and mass loss exist in practically every kind of star and have led to the discovery of accretion disk phenomena connected with compact objects. A comprehensive picture of magnetic activity evi denced by chromospheres, coronae, and extended atmospheres has been developed for large numbers of cool stars. Study of comets revealed new molecules; spectral observations of Comet Halley complemented the imaging from an international fleet of spacecraft and confirmed the source of ultraviolet emissions discovered by Pioneer Venus. Planetary aurorae associated with Jupiter, Saturn, and Uranus have been monitored. INFRARED ASTRONOMICAL SATELLITE The IRAS mission launched into polar sun-synchronous orbit was the result of a cooperative effort among the United States, the Netherlands, and the United Kingdom. IRAS made a survey of the entire sky at wavelengths of 12, 25, 60, and 100 J.,tm. The primary products of the mission were a catalog of 245,000 point sources (stars, galaxies, and star formation regions) and a set of 2 12 images covering the entire sky with 2 arc minute resolution in the four wavelength bands. This cata log has been widely distributed to astronomy research centers, and more detailed study is now possible largely through a Guest Observer program comprising peer-reviewed proposals. Studies of the data show that a significant number of nearby dwarf stars have a far infrared excess due to an extended disk or shell of solid material orbiting around and heated by the central star. This solid material is believed left over from the star formation process and represents protoplane tary material. Very small amounts of warm interstellar dust ("infrared cirrus") have also been discovered that can be used to trace out the local interstellar matter, to investigate the
Accomplishments of Astronomy and Astrophysics Explorers 22 jI OH II � I J 3 I I (It�] � '"' 2 'k "' I � n � 'H il � 0 0 I · ' .: .. .· I\ _ â¢' , ·"'----� ... · · r\ } .. ��--·_,v.....rr�Piof'".. � ... ... .... .. . ... ⢠· I y·�!. ·J:...· �.�·- ..:: _ ... __ , qI '---'-,___,.----;�:,.--,-...,...-: � I.� I Sz -- N"l.. I . . r â¢:x L� (b l I II I I ill - "I �� j -, � 'r "' 2� [ I !1� 1\ n1, I\v 'il v � . 1 I o rr..-l'j J .1.. - ,â¢,'( ·�·�t. .Jt_,'··,' .' l """' ., W ' ⢠I ,.,J,�, . . I I ⢠⢠1',;â¢:"·',,â¢, :â¢Â·â¢,,, : ' 2500 2600 2700 2800 2900 3000 3100 3200 WAVELENGTH (A) JUE: Discovery of the molecule s2 in the nucleus of comet ffiAS-Aralti-Alcoclt in Hl83 the comet nucleus; the broken lines denote the solar spectrum. The IUE has been one (spectrum in lower panel). The upper panel shows the spectrum measured away from of the most productive or all astronomical satellites. The targets of IUE range from solar-system objects, such as this comet discovered by ffiAS, to active galactic nuclei and quasars. dust-to-gas ratio, and define extinction-free areas in the sky. Stars of one solar mass or less have been revealed in the process of formation in nearby molecular clouds such as Taurus. Some 20,000 spiral galaxies were detected by IRAS. Many of these emit 5 to 1,000 times more energy in the infrared than at visual wavelengths. Some of these are galaxies harboring recent episodes of star formation ( "starburst" galaxies ), others may
Accomplishments of Astronomy and Astrophysics Explorers 23 emit copious amounts of infrared radiation due to the presence of dust-embedded active nuclei. IRAS has also contributed sig nificantly to the study of comets, asteroids, and dust within the solar system. ASTROPHYSICS EXPERIMENTS ON OTHER MISSIONS In addition to the Explorers that were devoted predom inantly to astronomy and astrophysics, a number of astrophy sics experiments have been carried out aboard Explorers that concentrated on other areas of space research. 1. Cosmic-Ray Research Cosmic-ray investigations provided some of the earliest astrophysics results from the Explorer Program. A series of experiments carried aboard Interplanetary Monitoring Plat forms (IMP) 1 through 8 (designated Explorers 1 8, 21, 28, 34, 41, 43, 47 and 50, respectively) made fundamental contribu tions to the study of cosmic ray spectra and composition. In the 1960s they demonstrated that the relative abundances of major elements are similar in galactic cosmic rays and in solar system matter, leading to the conclusion that the relativistic charged particles that permeate the galaxy are formed by nor mal processes of stellar nucleosynthesis. The most recent experiments have resolved the individual isotopes of some of the elements and have found significant excesses (relative to solar system matter) of neutron rich isotopes of the elements neon, magnesium, and silicon. These excesses indicate the pos sibility of regions of the galaxy with sizable metallicity enhancements or, alternatively, suggest a connection between cosmic ray injection and objects with peculiar surface abun dances, such as Wolf-Rayet stars. Experiments aboard the IMP satellites discovered the low energy "anomalous com ponent" of cosmic rays, now widely believed to be a population of particles accelerated from the local interstellar gas, and they demonstrated that this material does not contain the products
Accomplishments of Astronomy and Astrophysics Explorers 24 of nuclear fragmentation that are characteristic of galactic cosmic rays. Explorer-borne experiments also measured the abundances of several long lived radioactive isotopes, which were used as "clocks" to establish that cosmic rays are confined by the galactic magnetic field for approximately 1 0 million years. 2. Gamma-Ray Research Gamma-ray detectors were carried on several satellites of the IMP series and made fundamental discoveries concerning the surprising gamma-ray bursts. IMP-6, with detectors sensi tive to the 0.1-1 MeV energy range provided the first verifica tion of the gamma-ray burst phenomenon and obtained the first spectral measurements of gamma-ray bursts. Instruments on IMP-7 measured the spectra of about 25 gamma-ray bursts and demonstrated their similarity when averaged over the burst duration. ISEE-3 also carried instruments for the study of cosmic gamma-ray bursts. These instruments, used separately and in conjunction with experiments carried by an international network of spacecraft, made important contribu tions to the study of burst source spectra and locations.
IV. CURRENT AND FUTURE ASTRO NOMY AND AS TROPHYSICS EXPLORERS THE CURRENT PROGRAM The Announcements of Opportunity {AO) 6 and 7 issued in 1974, and selections in the mid-1970s, led to most of the current Explorers in the queue. Three astronomy missions are still waiting to be completed and launched : the Cosmic Back ground Explorer {COBE), the Extreme Ultraviolet Explorer (EUVE) and the X-Ray Timing Explorer (XTE). These mis sions are discussed below along with other projects now being funded under the Explorer line. 1. Cosmic Background Explorer The COBE, which is under construction and expected to be ready for launch in 1989, is designed to make a substantial improvement in our knowledge of the condition in the universe at large red shifts. The focus of the mission is to study the spectrum, intensity, and isotropy of the primeval cosmic back ground radiation at wavelengths longer than that of the 3 K black body radiation maximum {at 1 mm), and to detect or set stringent limits on the universal radiation energy density at shorter wavelengths-radiation emanating from distant sources but at later times than the primeval fireball. This mission will 25
